Production of thin goss oriented magnetic materials



United States Patent 3,165,428 PRODUCTIGN 6F THIN GGSS ORIENTED MAGNETEQ MATEREALS Paul A. Albert, Penn Hilts, and Karl Foster, Wilkins Township, Allegheny County, Pa assignors to Westinghouse Electric orporatien, Pittsburgh, Pa, a corporation of Pennsylvania No Drawing; Filed Dec. 27, 1962, Ser. No. 247,531 13 Ciairns. (Cl. '1i811l) This invention relates to the preparation of magnetic ferrous alloys that contain silicon and in particular it is concerned with the development in such alloys of large secondary grains oriented to provide a cube-on-edge or Goss texture in the resulting product.

This application is a continuation in part of our application Serial No. 781,938 filed December 22, 1958.

The cube-on-edge or Goss texture can be defined as a high degree of preferred alignment in the grains of a polycrystalline cube material wherein the unit crystallographic cells are aligned with a face diagonal and a cube edge parallel to the sheet surface. in addition, the cube edge is also parallel to the direction of rolling. Consequently, the orientation of such cellsis (1l0)[001]. In many cubic ferromagnetic materials, such as iron silicon alloys, the cube edge is also a direction of easy magnetization. Accordingly, a sheet of magnetic material having cube-on-edge texture presents a high permeability in the direction of rolling and such a characteristic is very desirable in a. number of magnetic applications. Such magnetic sheet is often designated as singly oriented.

Sheet material with a texture or preferred'orientation in a single direction as just described has been a product of commerce for many years in ferrous alloys containing 2.5 to 3.5 Weight percent or" silicon. These products have been obtained in thicknesses to about 15 mils. The methods employed in producing the Goss texture are highly dependent on alloying, sheet thickness and surface conditions.

In addition to those limitations, it has also been noted that in the thicker materials, that is those materials from 12 to 16 mils in thickness, the oriented grains have been of the type developed upon secondary recrystallization and the diameters of those grains generally has exceeded the sheet thickness by at least one order of magnitude. On the other hand, the oriented grains in the thinner materials, i.e., those that are from about 1 to 6 mils thick, rarely have a diameter as much as twice the sheet thickness and the grains are characteristically a product of primary recrystallization. The process employed commercially for producing thin gauge singly oriented silicon iron sheets is that set forth in Littmann Patent 2,473,156.

The primary recrystallized grain texture has magnetic properties inferior in important characteristics of initial permeability and coercive force. The sheet thickness has been found to be a critical factor in limiting growth of cube-on-edge grains by secondary recrystallization using processes known heretofore.

It is, therefore, a primary object of the present invention to provide a process, for the production of silicon containing ferrous alloys in thin sheet and tape having a thickness of 6 mils and less in which a cube-on-edge preferred orientation is obtained, that can be applied to 'al loys having a wider range of silicon content than heretofore; that can be applied to alloys containing significant elements in addition to silicon and iron; that results in large grains developed by secondary recrystallization during final anneal of the sheet; and that is simple and can be practiced with available equipment and skills.

Another object of the invention is to provide a novel thin gauge sheet below 6 mils in thickness of silicon- $305,428 Patented Jan. 12, .1965

iron alloy having from 2% to 8% silicon, characterized by a high volume proportion of large secondary grains having a (110) [001] orientation and of a diameter averaging at least 10 times the sheet thickness.

In accordance with the present discoveries, these and other objects are attained upon applying to a ferrous alloy containing silicon in an amount of about 2 to 8 percent, and preferably from 2.25% to 6.8% of silicon, a series of steps involving a final cold reduction of the alloy in an amount of about 40 to 95 percent and resulting in a sheet thickness of about 0.1 to 6 mils. Thereupon the individual uncoated sheet is treated by heating it, preferably by passing an electric current through it, while the alloy sheet is maintained uncovered in a non-oxidizing atmosphere, preferably in dry hydrogen or a vacuum, to heat the strip to at least 1100" C. under conditions whereby sulfur and oxygen are rapidly reduced to extremely low levels and secondary recrystallization of (110) [001] grains occurs in the sheet. During the final anneal the conditions should favor rapid evolution of sulfur and oxygen from the surface of the sheet and escape to the ambient atmosphere and removal from the vicinity of the sheets to attain a condition where less than 0.001% of sulfur and oxygen are present.

As a consequence of this procedure, substantially complete secondary recrystallization is attained in the alloy sheet and the resulting cube-on-edge oriented grains have diameters that are of a magnitude from at least one order greater to as much as a thousand times that of the sheet thickness. As least 70% of the sheet volume comprises grains having (110}[001] orientation, and usually to or more of the sheet volume comprises these grains. The resulting products are characterized by the outstanding magnetic properties of a low coercive force and a high maximum permeability that is evidenced in the thicker commercial material having the Goss texture.

The materials treated in accordance with this invention are ferrous alloys containing about 2 to 8 percent by weight of silicon, and preferably 2.25 to 6.8 percent silicon, the latter range involving alloys that are more readily worked. In addition such metallic elements as manganese, nickel, molybdenum, chromium, cobalt, copper and the like, either alone or in combination, to a total of about 0.1 to 1 weight percent can be used without preventing growth of the grains. Aluminum should not exceed 0.1%. Carbon should be less than 0.01%, preferably below 0.005%. Sulfur and oxygen are critical components in the sheet alloy. For sheets having a final gauge thickness of about 2 to 6 mils, the sulfur and oxygen both should be 0.001% or less at the time the sheet is subjected to the final anneal. As the sheets are thinner, 1 mil and less for instance, ordinary commercial quality silicon steel with the usual oxygen and sulfur content may be employed since during the final anneal the oxygen and sulfur will be rapidly reduced to levels of about 0.001% and less. Preferential growth of cube-on-edge nuclei takes place under such extremely low sulfur and oxygen content.

Commercially available iron and silicon can be used in the practice of this invention. A variety of commercially available relatively pure irons have been suc cessfully used without regard to source or the nature of impurities that were present. For example, electrolytic flake iron that was of a purity on the order of 99 percent has been used in many of the experiments. Suitably the alloys are prepared by melting the iron and silicon in a vacuum or in an inert atmosphere to protect the resulting alloy from undesirable contaminants. Typical gases that can be used to provide an inert atmosphere include hydrogen, helium, argon and the like and mixtures of such gases. The melt also is poured under such a protective atmosphere. The resulting alloy plate can then be reduced in preliminary steps to sheet or tape of about to 125 mils thickness which is then used in practicing the final steps of the present invention. Various combinations of hot and cold rolling, the latter with or without intermediate anneals, can and have been used to reduce the alloy plates to the preliminary sheet material for this invention. However, hot rolling is the most convenient method of bringing about this reduction rapidly to a plate or strip suitable for cold rolling to gauge. Starting material can also be supplied in other ways. For example, where a supply of the commercial, 12 to 16 mil thick Goss textured material is available and thin products, as in this invention, are desired, the thick material itself can be treated with the cold rolling and annealing schedule hereinafter described.

The alloy sheet, however obtained, is then cold rolled to the final thickness. The cold rolling is continued until a 40 to 95 percent reduction has occurred and the resulting product has a thickness of about 0.1 to 6 mils. Cold rolling can be carried out with a conventional rolling mill, such as a Sendzimir or Rohn mill; generally the reduction per pass is as large as is possible without experiencing cracking or the like so that the desired proddew points of .below 40 C., can be used as the annealing atmosphere. 'The atmosphere can be static or flowing, at the convenience of the operator, and is not preheated before use in the annealing step. If the gas is heated, it is preferred that it be at least 100 C. cooler than the sheet. If desired, the sheet can be annealed in a vacuum furnace at a high vacuum, for example at a pressure of below about 5 microns of mercury. The surface of the sheet should be free to emit gases such as oxygen and sulfur vapors and have them escape rapidly. The furnace or other enclosure should be relatively colder, in no event over 1050 C., and preferably colder by at least about 500 C. than the sheet at full annealing temperature so that impurities will tend to escape from the sheet to the walls.

The sheets at final gauge should not be coated with any of the usual coating or insulating compositions. The surface should be completely exposed to enable the oxygen and sulfur to be evolved. For similar reasons stacking or coiling of the sheet for placing the sheet in the furnace for the final anneal is not acceptable practice.

Annealing is conducted in accordance with-a desirable mode of this invention by passing an electric current through the sheet, as by physically connecting it in an AC. or D.C. circuit, or by subjecting it to an induced current. The use of current applied by making the sheet a-physical part of a circuit is preferred, for continuous operation without masking or covering the sheet surface i 7 Walls and atmosphere at'the same rate, so that those walls and atmosphere are cold compared to the sheet ternperature. Other methods forheating the shcetrare electron beam or electron cloud heating, or any high temperature radiation source.

7 growth occur quite rapidly in, for example, about /2 to 5 or 10 minutes. annealing condition for periods beyond that necessary for the recrystallization to occur, but no particular advantage results. Accordingly, the current can be turned off immediately after the secondary recrystallization occurs and the sheet is then furnace cooledto handling temperature. e p

The invention-will be described further by means of the following examples in which the details are givenby wayof illustration and are not to be construed as limiting the invention. a

' Example. I

Electrolytic flake iron' was melted in a Balzers vacuum melting induction furnace that was evacuated to 0.1 micron of mercury. About 3.5 parts, based on the iron, of commercial grade silicon (98%Si, balance essentially iron) was added to the melt. When the melting of the silicon was observed, an atmosphere-of helium was added to the furnace. The melt was stirredby induced current to insure homogeneity. Thereafter it was poured into a steel mold and solidified- The resulting plates were hot rolled at 1000 C. to 0.100 inch thick sheets. The sheets were then coldrolled to a final thickness of about 5 mils, a reduction of These tapes were suspended in a furnace whose walls were below 100 C. in which there was maintained an atmosphere of 5 0% dew point helium admitted at room temperature. The ends of the tape were connected to leads from a 115 volt A.C. source. The current was turned on and the tape temperature promptly brought to 1200 C., This condition was maintained for 3 minutes. Thereupon the current was cut olf, and the tape permitted to cool in the furnace to room temperature. The tape comprised substantially all secondary grain having [001] orientation, whose diameters exceeded 10 times the sheet thickness, many of the grains having diameters of 1000 times the'sheet thickness. Magnetic tests of the annealed sheet showed it had a coercive force of about 0.02 oersteds.

Example 11 The cold rolled sheet of Example I was finally annealed under similar conditions in a vacuum of approximately one micron. The annealed sheet was substantially completely secondarily recrystallized into grains having (110) [001] orientation. 7

Sheets of silicon-iron material of a final thickness of from 1 to 5 mils were produced in accordance with the foregoing Examples I and II and these comparable sheets have been subjected'to routine magnetic tests and crystallographic' study. I The sheets show a low coercive force tacts, the current supplied from the source and the speed of the sheet are then adjusted to permit each portion of the sheet to be in'the circuit fora period sufiicient to bring the portion to annealing temperature and to bring about complete secondary recrystallization. The speed and spacing, of course, depend on the current applied and the sheet thickness, andchen'ce will vary with those parameters. It can be noted that the manner of heating the alloy sheet doesnot heat the surrounding furnace of from 0.02 to 0.1 oersted, the lower-values being exhibitedby the thicker sheets, and high permeability at high induction and low fields that is characteristic of Goss texture. The permeability at H at 60 cycles exceeded 1800 and approached 1900. These values exceed the permeability of primary recrystallized texture by an average of at least 5%. Crystallographic study shows that nearly, 100% of sheet volume comprised grains having (110)[00l-}, orientation, that is, the Goss texture. Measurements on the grains have shown a range of sizes from twice the sheet thickness to a very. large size estimated to be about 1000 times the sheet thickness. These The sheet can be maintained at the large grains are clear evidence of secondary recrystallization, and they extend substantially throughout the entire sheet.

As a further example of the practice of the invention, commercial 14 mil 3% silicon iron sheet having a (110) [001] grain texture, is pickled to remove all the surface film. The clean sheet is cold rolled to a thickness of 1 mil. The thin sheet is individually vacuum annealed in a cold furnace, as in Example II, for 5 minutes to cause the oxygen and sulfur to be reduced to a low value and the thin sheet is allowed to cool in the furnace. The sheet will exhibit over 70% of its volume as comprising secondary grains having 110) [001] grain texture and excellent magnetic properties.

From the foregoing it is apparent that the present invention comprises a very efficient method of producing a cube-on-edge, (l)[001], orientation in thin sheet and wherein the resulting oriented grains or crystals are a product of secondary recrystallization. The process is effective without the need to rigidly control alloying constituents, surface conditions and the like as has been essential heretofore. Insofar as we are aware, this is the first successful preparation of sheet and tape of a thickness below six mils in which the preferred orientation (110) [001] coupled with substantially complete secondary recrystallization has been attained. The resulting products can be used in transformer, amplifier, relay applications and similar electrical applications.

In accordance with the provisions of the patent statutes, the present invention has been explained and there has been described what is now believed to represent its best embodiment. However, it should be understood that the invention can be practiced otherwise than as specifically exemplified.

We claim as our invention:

1. In the process of producing a magnetic sheet of a ferrous base alloy containing from 2% to 8% silicon and of a thickness of from about 0.0001 to 0.006 inch, the sheet having a major proportion of its volume composed of secondary recrystallized grains having (110) [001] orientation, the grains having an average diameter of at least ten times the thickness of the sheet, the steps comprising (1) cold reducing a thick sheet of the alloy at least 40% to a final gauge thickness of from about 0.0001 to 0.006 inch, the surface of the sheet being clean and free from any oxide and coatings,

(2) open annealing the uncoated cold reduced sheet at a temperature of from l100 C. to 1400 C. in a non-oxidizing atmosphere under furnace conditions whereby impurities in the sheet are readily evolved from the surface so that the oxygen and sulfur in the sheet are reduced to a value of below 0.001% each by the time secondary recrystallization begins, and

(3) continuing the annealing until substantially complete secondary recrystallization of the sheet occurs, the secondary grains substantially all having (110) [001] orientation.

2. In the process of producing a magnetic sheet of a ferrous base alloy containing from 2% to 8% silicon and of a thickness of from about 0.0001 to 0.006 inch, the sheet having a major proportion of its volume composed of secondary recrystallized grains having (110) [001] orientation, the grains having an average diameter of at least ten times the thickness of the sheet, the steps comprising (1) cold reducing a thick sheet of the alloy at least 40% to a final guage thickness of from about 0.0001 to 0.006 inch, the surface of the sheet being clean and free from any oxide and coatings,

(2) open annealing the uncoated cold reduced sheet by passing an electrical current through the sheet whereby the sheet is at a temperature of from 1100 C. to 1400 C. in a non-oxidizing atmosphere the walls of the annealing furnace being at temperature at least 500 C. cooler than the sheet whereby impurities in the sheet are readily evolved from the surface so that the oxygen and sulfur in the sheet are reduced to a value of below 0.001% each by the time secondary recrystallization begins, and (3) continuing the annealing until substantially complete secondary recrystallization of the sheet occurs, the secondary grains substantially all having (110) [001] orientation. 3.. The process of claim 2, wherein the annealing atmosphere comprises hydrogen gas of a dew point of below '40 C. and the hydrogen is at a temperature of at least 100 C. cooler than the sheet.

4. The process of claim 2 wherein the annealing is carried out in a vacuum of less than 5 microns absolute pressure.

5. The process of claim 2 wherein the electrical current is applied to the sheet from spaced contacting electrodes connected to a source of electrical current, and the sheet is moved relative to the contacting electrodes at a rate such that each portion the sheet is at a temperature of from 1100 C. to 1400 C. for a period of time to attain substantially complete secondary recrystallization.

6. In the method of producing magnetic sheet material of a thickness of about 0.001 to 0.006 inch of a ferrous alloy containing about 2 to 8 percent of silicon characterized by relatively large grains having cube-on-edge, (110) [001], orientation, the steps comprising finally cold reducing sheet of the alloy 40 to 95 percent to a thickness of from about 0.001 to 0.006 inch and then finally annealing the resulting cold rolled sheet while uncoated and exposed and in a non-oxidizing atmosphere by passing electrical current through the sheet in an amount sufficient to heat it promptly to a temperature of from 1100 C. to 1400 C. for a period of from about /2 to 5 minutes in a furnace whose walls are cooler than the sheet whereby oxygen and sulfur in the sheet are rapidly reduced to less than 0.001% before secondary recrystallization occurs, the heating being applied until substantially complete secondary recrystallization occurs and produces grains whose average diameter exceeds twice the sheet thickness.

7. A method in accordance with claim 6 in which said alloy contains about 2.25 to 6.8 percent of silicon.

8. A method in accordance with claim 6 in which said non-oxidizing atmosphere comprises a vacuum of below about 5 microns absolute pressure.

9. A method in accordance with claim 6 in which said non-oxidizing atmosphere comprises a dry gas of a dew point of below 40 C. substantially inert to said alloy sheet.

10. In a method of producing magnetic sheet material of a thickness of about 0.00l to 0.006 inch of a ferrous alloy containing about 2 to .8 percent of silicon characterized by relatively large grains having a cube-on-edge, (110) [001], orientation, the steps comprising finally cold reducing sheet of the alloy 40 to percent to a thickness of from about 0.001 to 0.006 inch and then finally annealing the resulting cold rolled sheet in a furnace in a non-oxidizing atmosphere by passing the sheet into contact simultaneously with two spaced conductors connected to a source of current whereby the portion of said sheet between said conductors is heated promptly to a temperature of about 1100 C. to 1400 C., the furnace walls being at a temperature of below 1050 0., whereby impurities are evolved from the sheet and moving said sheet relative to said conductors at a rate adapted to permit each portion of the sheet to be in the resulting circuit for a period sufi'icient to cause substantially complete secondary recrystallization therein and produces grains whose average diameter exceeds twice the sheet thickness.

11. A method in accordance with claim 10 in which said non-oxidizing atmosphere comprises a cold flowing gas having a dew point of below 40 C. and inert to said alloy.

12. A magnetic sheet of a thickness of from about 00001 to 0.006inchof an iron-silicon alloy consisting essentially of from 2.25% to 6.8% by Weight of silicon sulfur less than 0.001% and a maximum of about 0.001%.

oxygen, and the balance being iron except for metallic additions and impurities not, exceeding about 1% by weight, of which aluminum does not exceed 0.01% by weight, the-sheet having at least 70% of its volume composed of secondarily recrystallized grains having (110) [001] orientation, the secondary grains having an average diameter of at least ten times the sheet thickness.v

of secondary recrystallized grains. having (110) [-001] orientation, thegrains having an average diameter of at least-ten times the thickness of the sheet.

References Cited in vthe file of this patent UNITED STATES PATENTS 2,473,156 Littmann "June 14, 1949 2,535,420 Jackson Dec. 26, 1950 2,765,246 Maxwell Oct.2, 1956 2,867,559 May. Jan. 6, 1959 FOREIGN PATENTS 639,252

Great Britain June 28, 1950 

1. IN THE PROCESS OF PRODUCING A MAGNETIC SHEET OF A FERROUS BASE ALLOY CONTAINING FROM 2% TO 8% SILICON AND OF A THICKNESS OF FROM ABOUT 0.0001 TO 0.006 INCH, THE SHEET HAVING A MAJOR PROPORTION OF ITS VOLUME COMPOSED OF SECONDARY RECRYSTALLIZED GRAINS HAVING (110)(001) ORIENTATION, THE GRAINS HAVING AN AVERAGE DIAMETER OF AT LEAST TEN TIMES THE THICKNESS OF THE SHEET, THE STEPS COMPRISING (1) COLD REDUCING A THICK SHEET OF THE ALLOY AT LEAST 40% TO A FINAL GAUGE THICHKNESS OF FROM ABOUT 0.0001 TO 0.006 INCH, THE SURFACE OF THE SHEET BEING CLEAN AND FREE FROM ANY OXIDE AND COATINGS, (2) OPEN ANNEALING THE UNCOATED COLD REDUCED SHEET AT A TEMPERATURE OF FROM 1100*C. TO 1400*C. IN A NON-OXIDIZING ATMOSPHERE UNDER FURNACE CONDITIONS WHEREBY IMPURITIES IN THE SHEET ARE READILY EVOLVED FROM THE SURFACE SO THAT THE OXYGEN AND SULFUR IN THE SHEET ARE REDUCED TO A VALUE OF BELOW 0.001% EACH BY THE TIME SECONDARY RECRYSTALLIZATION BEGINS, AND (3) CONTINUING THE ANNEALING UNTIL SUBSTANTIALLY COMPLETE SECONDARY RECRYSTALLIZATION OF THE SHEET OCCURS, THE SECONDARY GRAINS SUBSTANTIALLY ALL HAVING (110) (001) ORIENTATION. 